Downloadable PDFs!
Dialect is an important part of writing character dialogue. It helps keep characters vivid and real, and is much less annoying than substituting apostrophes for letters or getting out the punctuation shaker. The dialect worksheet prompts or cues are described herein, but you can also download one of these guides for more portable help.
Dialect Worksheet - This includes and uses the descriptions indexed below.
Conlang Cheat Sheet - Since we can't all be Tolkien, some very basic questions to help you make an originally constructed language if all you want to do is throw in half a scene of dialogue in a made-up language or some lines scattered throughout your work.

Word Substitutions Word substitutions are the easiest and simplest ways to create dialect, through jargon and replacing basic terms with ones specific to certain fields of interest, professions or hobbies or academic fields. Alternatively or in conjunction, there are localized word substitutions based on geographic region, the most famous one in the US being where the lines are between soda, pop, and cola. When creating a fantasy or other spec-fic world, most commonly people use simple one-for-one word swaps, usually for curses or slang.

Examples:
Soda/Pop/Cola
Shiny/Cool
Frak/Frell/You know

Word additions
Languages are versatile and mutable and very often hungry. Japanese takes words from German, English takes words from French, Russian takes words from English, and it goes around and around. When creating speculative fiction involving cultures with conlangs, most authors tend to both only create limited phrases rather than entire conlangs, and most authors subscribe to either the strong or weak Sapir-Whorf hypothesis/theory of linguistic relativity, which states that language either determines or influences cognition, thought.

Examples:
schadenfreude (German to English, joy in the discomfort of others)
arubaito (German to Japanese, part-time work)

Emphatics
Emphatics tend to differ from language to language, but they come most often in one of several forms. Repeating words for emphasis is most famously exemplified in pidgin, forming compound such as big-big or many-many (or many-lots, in Troll). Sometime there are phrases tacked on for emphasis or confirmation, such as the habit of appending a sentence with "yes?" to encourage agreement. Suffixes are most commonly used in English to increase the degree of an adjective, or sometimes entirely separate words. Then, sometimes, there are phrases...

Phrases
A lot of slang takes single word form as a word substitution, but a lot of slang also takes phrase form. We know many of these on the internet as memes.

Examples:
"back your play" or "watch your back" or "watch your six" or "back me up"

Grammatical Quirks
This one is trickier if you don't have a working knowledge of grammar: a lot of dialect can be put down to grammatical tics. Some of the structural differences that are typical of the speaker(s) might be due to a different native language. (Usually these are genitive noun structures or verb structures.) (Or prepositions. Fucking prepositions.) Others are simply common differences between grammatically correct language and language as it is actually used.

Foreign Language Translation Tics
One of the most well-known examples of the foreign language translation tic is the Russian lack of articles. This is actually true; there are no indefinite or definite articles in Russian, although there are indicatives. Remembering to put in extra parts of speech that don't exist in your native language can be difficult, so these are usually one of the first indicators. Prepositions never, in my experience, match one-to-one for all potential usages. One preposition in one language will be used for six different situations, and two of those will match up to another preposition in another language which will also be used for two more situations that do not match the original language. If it sounds confusing it's because it is. In Irish, the reflexive version of pronouns is often used for emphasis or to refer indirectly to a particular person, etc., according to context. In English this may transfer to only the reflexive.

"'Tis himself that's coming now." Is é féin atá ag teacht anois.

Word Drop/Word Add
Obviously, we don't speak the way we write, and we don't even write with proper grammar all the time. In speech more than in writing, we tend to drop words or add words, depending. The words most commonly dropped are ones that are understood from context, such as copula verbs (linking verbs) or pronouns. The words most commonly added tend to be words that double as flavoring particles, such as in English 'like', 'and', or 'so'

"[It's] Got its disadvantages."
"[Do/Can] You see your way to a finish, yet?"
"So, of course, vampires can be driven away with a cross, so, we're all wondering: do fingers work?"

Appositives
An appositive phrase is a noun phrase that restricts, specifies, or enhances a noun. Apart from adding words to your count, it can add flavor to your sentences and indicate a person's point of view or state of mind.

"As to the trial, that takes place Wednesday next, I suggest..."